Inorganic compounds are used as raw materials or final products in various fields and also used as raw materials of electrode active materials for secondary batteries, which have recently been increasingly used.
Lithium secondary batteries, which are a representative example of secondary batteries, generally use lithium cobalt oxide (LiCoO2) as a cathode active material, a carbon material as an anode active material, and lithium hexafluorophosphate ((LiPF6) as an electrolyte. As the cathode active materials, layered LiCoO2, lithium nickel oxide (LiNiO2), spinel lithium manganese oxide (LiMn2O4), and the like are known, but LiCoO2 is the most commonly used in commercial applications.
However, supply of cobalt as a main component is unstable and cobalt is expensive and thus materials, some cobalt atoms of which are substituted with a transition metal such as Ni, Mn, or the like, or spinel LiMn2O4 and the like which contain very little cobalt have begun to be commercially used. In addition, novel compounds with more stable structure even at high voltage or materials that are prepared by doping or coating existing cathode active materials with other metal oxides and thus have enhanced stability have been developed.
Among conventional methods of preparing cathode active materials, dry calcination and wet precipitation are mostly widely known methods. According to dry calcination, a cathode active material is prepared by mixing an oxide or hydroxide of a transition metal such as cobalt (Co) or the like with lithium carbonate or lithium hydroxide as a lithium source in a dried state and then calcining the resulting mixture at a high temperature of 700° C. to 1000° C. for 5 to 48 hours.
Dry calcination is, advantageously, a widely used technology for preparing metal oxides and thus is easy to approach, but is disadvantageous in that it is difficult to obtain single-phase products due to difficulties in uniform mixing of raw materials and, in the case of multi-component cathode active materials consisting of two or more transition metals, it is difficult to homogeneously arrange at least two elements to atom levels. In addition, when a method of doping or substituting with particular metal components to improve electrochemical performance is used, it is difficult to uniformly mix the particular metal components added in small amounts and loss of the metal components inevitably occurs through pulverizing and sorting processes performed to obtain desired particle sizes.
Another conventional method of preparing cathode active materials is wet precipitation. In wet precipitation, a cathode active material is prepared by dissolving a salt containing a transition metal such as Co or the like in water, adding alkali to the solution to precipitate the transition metal in the form of transition metal hydroxide, filtering and drying the precipitate, mixing the resulting precipitate with lithium carbonate or lithium hydroxide as a lithium source in a dried state, and calcining the mixture at a high temperature of 700° C. to 1000° C. for 1 to 48 hours.
The wet precipitation method is known to easily obtain a uniform mixture by co-precipitating, in particular, two or more transition metal elements, but requires a long period of time in precipitation reaction, is complicated, and incurs generation of waste acids as by-products. In addition, various methods, such as a sol-gel method, a hydrothermal method, spray pyrolysis, an ion exchange method, and the like, have been used to prepare a cathode active material for lithium secondary batteries.
In addition to the methods described above, a method of preparing an inorganic compound for a cathode active material by hydrothermal synthesis using high-temperature and high-pressure water is used.
With regards to this, referring to FIG. 1, in a conventional hydrothermal synthesis device 10, precursor solutions are respectively supplied from upper and side parts of a mixer 20 via supply tubes 22, 22a and 22b, the supplied precursor solutions are mixed to prepare an intermediate slurry f1 and then the intermediate slurry f1 is supplied to a reactor 11 via a connection tube 30 connected to the reactor 11, and, while supercritical liquid streams containing high-temperature and high-pressure water are injected from opposite sides of the reactor 11, reaction between the intermediate slurry f1 and the supercritical liquid streams occurs in the reactor 11 for a short period of time.
In this regard, the intermediate slurry f1 supplied to the reactor 11 has increased shearing stress over time due to the viscosity of the intermediate slurry f1 and friction on an inner surface of the connection tube 30 and thus does not smoothly move, which results in accumulation of the intermediate slurry f1 on the inner surface thereof. In addition, reaction of the intermediate slurry f1 accumulated at an entrance portion of the reactor 11 occurs, thus causing clogging of the entrance portion thereof.
In addition, when the temperature of the connection tube 30 is increased due to high temperature (about 400° C.) of supercritical water, solubility of an inorganic material included in the intermediate slurry f1 is reduced and thus the inorganic material is deposited onto a surface of the connection tube 30, which results in clogging of the connection tube 30.
Consequently, a continuous operating time of a hydrothermal synthesis device is only about 1 week, and much labor and time are required for disassembly and internal cleaning of the clogged reactor.
Therefore, there is a high need to develop a continuous hydrothermal synthesis device in which a continuous operating time is increased by minimizing clogging of an inlet and thereby productivity may be significantly increased and investment costs may be reduced.